Delayed coking systems are commonly used in petroleum refineries for converting vacuum tower bottoms and/or other heavy (i.e., high boiling point) residual petroleum materials to petroleum coke and other products. The greater part of each barrel of resid material processed in the cocker will typically be recovered as fuel gas, coker gasoline/naphtha, light cycle oil (also commonly referred to by various other names such as light coker gas oil), and heavy cycle oil (also commonly referred to by various other names such as heavy coker gas oil).
A typical delayed coking system comprises: a combination tower or other fractionator; a fired heater; and at least a pair of vertical coking drums. The heavy coker feed is typically delivered to the bottom of the fractionator where it is combined with a heavy residual bottom product, commonly referred to as “recycle,” produced in the fractionator. The resulting mixture is drawn from the bottom of the fractionator and then pumped through the heater and into at least one coking drum. Typically, multiple coking drums are operated in alternating cycles such that, while one drum (referred to herein as the “live” drum) is operating in a fill cycle, another drum is operating in a second cycle typically comprising: a steamout to fractionator stage; a steamout to blow down stage; a cooling/quenching stage (which causes the coke to form a solid mass within the drum); a draining stage; a drum unheading stage; a hydraulic de-coking stage for cutting the solid coke mass into chunks; a reheading and pressure testing stage; and a warmup/preheating stage.
In the fill cycle, the hot feed material from the coker heater typically flows into the bottom of the live coking drum. Some of the heavy feed material vaporizes in the beater such that the material entering the bottom of the coking drum is a vapor/liquid mixture. The vapor portion of the mixture undergoes mild cracking in the coker heater and experiences further cracking as it passes upwardly through the coking drum. The hot liquid material undergoes intensive thermal cracking and polymerization as it remains in the coking drum such that the liquid material is converted to cracked vapor and petroleum coke. The resulting combined overhead vapor product produced in the coking drum is typically delivered to a lower portion of the fractionator. The cracked vapor product is typically separated by the fractionator into gas, naphtha, light cycle oil, and heavy cycle oil, which are withdrawn from the fractionator as products, and a heavy recycle/residual material which flows to the bottom of the fractionator. The light and heavy cycle oil products are typically taken from the fractionator as side draw products which are further processed (e.g., in a fluid catalytic cracker) to produce gasoline and other desirable end products. The heavy recycle material combines with the heavy feed material in the bottom of the fractionator and, as mentioned above, is pumped with the heavy feed material through the coker heater.
By way of example, but not by way of limitation, typical coker operating conditions and products specifications include: a coker heater outlet temperature in the range of from about 905° to about 935° F.; live coke drum pressures in the range of from about 20 to about 40 psig; live drum overhead temperatures in the range of from about 800° to about 820° F.; a fractionator overhead pressure in the range of from about 10 to about 30 psig; a fractionator bottom temperature in the range of from about 750° to about 780° F.; a light cycle oil draw temperature in the range of from about 450° to about 550° F.; a light cycle oil initial boiling point (ASTM D-1186) in the range of from about 300° to about 325° F.; a light cycle oil endpoint D-1186 in the range of from about 600° to about 650° F.; a heavy cycle oil draw temperature in the range of from about 600° to 690° F.; a heavy cycle oil initial boiling point (D-1186) in the range of from about 470° F. to about 500° F.; and a heavy cycle oil end point (D-1186) in the range of from about 960° to about 990° F.
There is currently a trend in the U.S. refining industry toward the processing of heavier, lower cost crudes. This results in refiners having to contend with much larger quantities of residual materials in the refining process. This, in turn, increases the demands on the refinery's residual conversion processes, especially delayed coking. Since the greater part of a barrel of residuum (such as, e.g., the high boiling point bottom products from atmospheric or vacuum distillation columns) can be converted to light ends, gasoline, distillate, and gas oil in a coker, the coker has become even more important in today's refinery economics.
Unfortunately, coking systems are often the principal bottleneck in many refineries when it comes to increasing refinery production rates and to improving product quality. The operation of a delayed coking system is a combination batch-continuous process. While one drum is live (i.e., is being filled with hot feed material), another drum is being stripped, quenched, decoked, and warned. Then, at the end of the filling cycle, the operation of the drums is switched. This cycle of events results in significant variations in the composition of the vapor feed to the fractionator over time and generates numerous operating problems associated with this type of operation, such as pressure swings, temperature swings, etc. Thus, the flow of feed and recycle material from the bottom of the coker fractionator to the coker heater, although continuous, is subject to considerable fluctuation due to the effect of drum switching operations and other factors which greatly influence fractionator stability. In addition, other problems commonly experienced in delayed coking systems include foaming, drum foam-over, inefficient liquid product recovery, inferior coke and liquid product quality, and extended drum cycle length.
The time required for drum filling and decoking operations in delayed coking systems has also severely limited the maximum achievable throughput for these systems. By way of example, the coking drums used in existing delayed coking processes will typically operate on about 18 hour cycles. Thus, while one drum is operating in an 18 hour filling cycle, another drum will undergo an 18 hour decoking cycle. A typical 18 hour decoking cycle involves: about 0.5 hours for the steamout to fractionator operation; about 1.0 hours for the steamout to coker blowdown operation; about 5.5 hours for the water quench/fill operation; about 2.0 hours for the quench water draining operation; about 0.5 hours for the drum unheading operation; about 3.0 hours for the decoking (i.e., hydraulic cutting) operation; about 1.0 hours for reheading the coking drum and conducting a pressure test to verify that the drum has not been damaged; and about 3.5 hours for warming the drum with steam to return it to operating temperature.
In addition to all of the other problems, shortcomings, and disadvantages discussed above, the specialized hydraulic decoking apparatuses required for use in the prior art delayed coking systems are very costly to obtain, install, and maintain. Moreover, the drum quenching and fill procedures required in the prior art processes waste tremendous amounts of heat and generate large volumes of waste water which must be processed in the refinery's wastewater treatment system. Further, the tremendous drum temperature swings experienced between the coking, quenching, and other stages of the prior art process, as well as the unheading and reheading of the coking drums for decoking, place tremendous stresses on the coking drums and create a significant potential for drum damage and downtime.
The delayed coking processes and systems heretofore used in the art are also limited in terms of the maximum heater outlet temperature which can be employed. The maximum coker heater outlet temperature employed in the present delayed coking systems generally cannot exceed 935° F. and most preferably will not exceed 930° F. The use of higher heater outlet temperatures results in the production of a very hard coke product which is very difficult to cut and remove. However, this maximum temperature limit prevents the resid feed material from being fully cracked. Consequently, some heavy liquid material remains in the green coke product. By failing to fully crack the resid feed and leaving some of the heavy liquid material in the coke product, the overall product yield is reduced and the coke product has an undesirable volatile organic carbon (VOC) content.
Consequently, a need exists for an improved delayed coking process and apparatus which alleviates or eliminates the various problems, limits, and shortcomings of the delayed coking processes and systems heretofore used in the art.